Hong Kong Geology Guide Book Geotechnical Engineering Office ...

1) Eroded into the cliff, to the west of
Swire Institute of Marine Science, Hong
Kong University, is a large sea cave that
makes an excellent starting point. The
roof of this sea cave has been breached,
so that it is now a blowhole. A blowhole
is narrow chamber in a sea cliff with an
opening at the top. At certain stages of
the tide, incoming waves funnel under
Blow hole with sea water rushing in
pressure into the cave so that sea water
and spray are pushed upwards and then
forced out through the relatively narrow opening in the roof. Closely examine the
geology and try to determine if there is any difference in rock types on opposite
sides of the cave. Read the geological map and try to determine how the cave was
formed.
Blow hole on the hillside Distinctive dark
lamprophyre dyke
Lamprophyre cutting
quartzphyric rhyolite
2) Walk westwards along the footpath to the northwestern shore of Cape D’Aguilar
Bay. There, you will see a mafic dyke cutting across the granodiorite. Carefully
observe the contact relationships between the dyke and the country rock. The
mafic dyke has a specific relationship with the blowhole, try to determine what
that is. There are many mafic dykes and veins in the area, some of which are only
several tens of centimetres in thickness.
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